This invention relates to a molded resin article and a method for the manufacture thereof. More particularly but not exclusively, it relates to a molded resin distributor rotor for an ignition distributor of an internal combustion engine and to a method of manufacturing the distributor rotor.
The distributor rotor of an ignition distributor for an internal combustion engine is generally formed by the injection molding of a thermoplastic resin. Due to air which is entrained with the resin during injection or due to sink marks which form during the cooling of the resin, voids inevitably form in the thick portions of the distributor rotor, such as in the central portion of the rotor which fits over the spark advance sleeve of the distributor. A distributor rotor is required to have excellent dielectric strength, and therefore these voids are highly undesirable, since they decrease the dielectric strength of the rotor. Furthermore, gas discharges may occur within the voids, which over a period of time may chemically and mechanically degrade the resin to the point of breakdown. In the worst case, a ground short can occur through the distributor rotor, adequate electrical energy for ignition can not be obtained, and the ignition is poor.
Various methods have been proposed for minimizing the number of such voids in a molded distributor rotor. For example, Japanese Utility Model Laid-Open No. 59-22294 (1984) discloses a method of molding a distributor rotor by injection molding in which the mold gate opens onto the outer periphery of the rotor along the balance portion thereof. Although that method disperses the voids within the rotor and reduces the diameters thereof, it can not completely prevent the formation of voids at the central portion of the rotor where the greatest dielectric strength is required, and therefore it is not completely satisfactory.